1. Field of the Invention
The present invention relates to a developing agent for electrostatic latent image development and, more particularly, to a non-magnetic yellow toner for use in full-color image forming apparatuses, such as full-color electrostatic copying machines and full-color laser beam printers.
2. Description of the Related Art
An image forming method in which an electrostatic latent image formed on an electrostatic latent image supporting member, such as a photosensitive member, is developed by using a toner, and in which the developed toner image is transferred onto a recording material, such as recording paper, for image formation has been widely used in copying machines, printers, facsimile, and the like. Recently, a full-color image forming apparatus has been put in practical use such that multicolor image reproduction can be made by superposing toners of plural colors one over another.
In such a full-color image forming apparatus, an electrostatic latent image is formed in dot units on an organic photosensitive member charged negatively by digital writing, for example, light beam irradiation, and the latent image is reversal-developed by using negatively chargeable magenta toner, cyan toner, yellow toner and, where necessary, black toner, and toner images of different colors are superposed one over another thereby to achieve multicolor image reproduction.
Above described full-color image forming method is largely employed in reproducing pictures, photographs, graphic images, and the like, and multicolor image reproduction is carried out by superposing color toners of plural colors one over another as stated above. Such a multi-color imaging method is used not only for image formation on recording paper, but also is generally adopted for use with an overhead projector sheet (OHP sheet). Therefore, it is required that toner must have a spectral reflectance corresponding to the desired color and good transmittance such that an underlying toner color is not concealed when color toners are superposed one over another. This requirement is particularly pronounced with respect to yellow toner, a toner of pale color.
Known organic pigments and dyes have been conventionally used as coloring agents for yellow toner. However, such pigments and dyes have their inherent shortcomings. In the case of dyes, for example, while they generally have high permeability and good colorfulness because the dye is present in such a condition that the dye is dissolved in the binding resin of the toner, they have a disadvantage that their resistance to heat and to light is rather low. The low resistance to light leads to fading due to light. Therefore, even when a highly colorful image is obtained, the problem is that the colorfulness cannot be long maintained. The low resistance to heat is apt to cause a problem such that when thermal fixation is carried out, the dye is sublimated in the vicinity of the fixing section, resulting in some interior stain. Another problem is that some dye is liable to become dissolved in a release agent, such as silicone oil, applied to the fixing roller, resulting in image stains or the like. In view of these problems, it is preferable to use pigments. However, pigments are so cohesive that it is difficult to finely and uniformly disperse the pigment in the toner, and as a result the hiding effect of the toner is so high as to lower the permeability of the toner itself. Another problem with pigments is that if agglomeration of pigment particles occurs, the agglomerated particles cause light scattering with the result that any sufficient spectral reflection for accurately reproducing a document cannot be obtained. Another problem is that since pigments have high hiding power as stated above, no sufficient permeability could be obtained. Therefore, when the toner is used with an OHP sheet, the projected image is dark and has poor chroma. Further, some pigments have low resistance to heat such that the pigment is decomposed during the process of toner preparation or during the stage of heat fixing.
Whilst, as earlier stated, full-color toners are required to have good transferability for reproducing a multi-color image by superposing toners of different colors one over another. If the charged amount of the toner is too high, the strength of toner adhesion to the photosensitive member tends to increase, resulting in lowered transferability of the toner. In such a case, even if the toner charge is broadly distributed, there will occur some degradation in transferability. With full-color toners, therefore, it is required that the toner must have a proper charge amount and a proper charge distribution. Such a requirement is particularly pronounced in the case where toners are transferred by means of a transfer roller onto an intermediate transfer member on which toners of different colors are superposed one over another so that the superposed toners are transferred by the transfer roller onto a transfer sheet.
Recently, size reduction has been considered with respect to full-color image forming apparatuses and, to this end, it is required that developing units must be reduced in size. The reason for this is that a full-color image forming apparatus requires four developing units for accommodating cyan developing agent, yellow developing agent, magenta developing agent, and black developing agent respectively. For the purpose of size reduction with respect to a developing apparatus, it is advantageous to use a non-magnetic mono-component developing apparatus which does not require an agitator mechanism for stirring the toner and carrier into mixture. With the non-magnetic mono-component developing apparatus in which no carrier is used, however, the toner is required to have a prompt rise behavior to quickly give a proper charge quantity, because the toner is charged through its contact with the developer supporting member and/or developer regulating member.